An immunoassay method is employed for measuring trace substances contained in blood, urine, and the like. Since the immunoassay method is based on a strong specific antigen-antibody binding, specific and high-sensitivity measurement of the objective substance is possible even from a sample containing various substances therein.
However, there has been a growing need for measurement of ultra-trace substances including: a cancer marker in the blood, antigens such as a virus; and antibodies against bacteria and viruses. As a result, a higher-sensitivity measurement has been strongly desired in the immunoassay method.
Conventionally-disclosed methods for achieving the higher-sensitivity measurement in the immunoassay method include a method of adding a reaction accelerator to a reagent (Patent Document 1), a method of adding a large amount of inactive protein to a reaction system (Patent Document 2), and a method of heat-denaturing a blocking agent when conducting blocking of antigens and antibodies immobilized on insoluble carriers with immunoinactive proteins (Patent Document 3).
However, the method of adding a reaction accelerator disclosed in Patent Document 1 has a problem that a non-specific reaction may be induced.
In the methods disclosed in Patent Documents 2 and 3, serum-derived albumin is commonly used as a blocking agent for preventing a non-specific reaction. However, serum albumin products have problems that the reactivity may vary widely from lot to lot and that a sufficient effect may not be obtained.    Patent Document 1: Japanese Kokai Publication H04-122858(JP-A H04-122858)    Patent Document 2: Japanese Kokai Publication 2000-46828(JP-A 2000-46828)    Patent Document 3: Japanese Kokai Publication H10-197530(JP-A H10-197530)